The present invention relates to a filter medium, and more particularly to a filter medium for use in an oil filter or an air filter for an internal combustion engine.
Filter mediums for use in oil and air filters for internal combustion engines for automobiles or the like are required to have such filtration efficiency as to effectively remove minute particles such as dust particles from a fluid being filtered and also to keep good filtering performance for a long period of time.
Various filter mediums have heretofore been proposed to meet the above requirements. However, the conventional filter mediums have proven unsatisfactory in achieving both increased filtration efficiency and longer service life.
There is known a filter medium formed of filter paper only, which filter paper comprises pulp, rayon and linter. In use, the filter medium composed of filter paper only forms on its surface a dust cake layer which has a filtering function (i.e., primal filtration function) for increased filtration efficiency.
The filter medium formed of the filter paper of this type is normally impregnated with oil in order to promote the formation of a dust cake layer on the surface thereof.
The oil impregnated in the filter medium, however, serves as a carrier for carrying the dust through the filter paper, with the result that the dust tends to be mixed with the fluid being filtered, thus lowering the filtering efficiency of the filter medium. Even if the dust does not pass through the filter paper, it is trapped by a deep layer in the filter paper (so-called "depth-type filtration"), causing the filter paper to be plugged or clogged, and consequently the pressure loss across the filter medium is increased.
During use of the filter medium over many years, the amounts of impregnated oil is reduced due to evaporation, and no sufficient formation of a dust cake layer is accomplished. Therefore, the primal filtering function is no longer performed by the dust cake layer, and the desired long service life cannot be achieved.
Another problem is that the oil impregnated in the filter paper is liable to be scattered around to clog a carburetor in the case where the filter medium is used in an air filter.